The present invention relates generally to micro-electromechanical devices and, more particularly, to micro-electromechanical thermal actuators such as the type used in ink jet devices and other liquid drop emitters.
Micro-electro mechanical systems (MEMS) are a relatively recent development. Such MEMS are being used as alternatives to conventional electromechanical devices as actuators, valves, and positioners. Micro-electromechanical devices are potentially low cost, due to use of microelectronic fabrication techniques. Novel applications are also being discovered due to the small size scale of MEMS devices.
Many potential applications of MEMS technology utilize thermal actuation to provide the motion needed in such devices. For example, many actuators, valves and positioners use thermal actuators for movement. In some applications the movement required is pulsed. For example, rapid displacement from a first position to a second, followed by restoration of the actuator to the first position, might be used to generate pressure pulses in a fluid or to advance a mechanism one unit of distance or rotation per actuation pulse. Drop-on-demand liquid drop emitters use discrete pressure pulses to eject discrete amounts of liquid from a nozzle.
Drop-on-demand (DOD) liquid emission devices have been known as ink printing devices in ink jet printing systems for many years. Early devices were based on piezoelectric actuators such as are disclosed by Kyser et al., in U.S. Pat. No. 3,946,398 and Stemme in U.S. Pat. No. 3,747,120. A currently popular form of ink jet printing, thermal ink jet (or xe2x80x9cbubble jetxe2x80x9d), uses electrically resistive heaters to generate vapor bubbles which cause drop emission, as is discussed by Hara et al., in U.S. Pat. No. 4,296,421.
Electrically resistive heater actuators have manufacturing cost advantages over piezoelectric actuators because they can be fabricated using well developed microelectronic processes. On the other hand, the thermal ink jet drop ejection mechanism requires the ink to have a vaporizable component, and locally raises ink temperatures well above the boiling point of this component. This temperature exposure places severe limits on the formulation of inks and other liquids that may be reliably emitted by thermal ink jet devices. Piezo-electrically actuated devices do not impose such severe limitations on the liquids that can be jetted because the liquid is mechanically pressurized.
The availability, cost, and technical performance improvements that have been realized by ink jet device suppliers have also engendered interest in the devices for other applications requiring micro-metering of liquids. These new applications include dispensing specialized chemicals for micro-analytic chemistry as disclosed by Pease et al., in U.S. Pat. No. 5,599,695; dispensing coating materials for electronic device manufacturing as disclosed by Naka et al., in U.S. Pat. No. 5,902,648; and for dispensing microdrops for medical inhalation therapy as disclosed by Psaros et al., in U.S. Pat. No. 5,771,882. Devices and methods capable of emitting, on demand, micron-sized drops of a broad range of liquids are needed for highest quality image printing, but also for emerging applications where liquid dispensing requires mono-dispersion of ultra small drops, accurate placement and timing, and minute increments.
A low cost approach to micro drop emission is needed which can be used with a broad range of liquid formulations. Apparatus and methods are needed which combine the advantages of microelectronic fabrication used for thermal ink jet with the liquid composition latitude available to piezo-electromechanical devices.
A DOD ink jet device which uses a thermo-mechanical actuator was disclosed by T. Kitahara in JP 2,030,543, filed Jul. 21, 1988. The actuator is configured as a bi-layer cantilever moveable within an ink jet chamber. The beam is heated by a resistor causing it to bend due to a mismatch in thermal expansion of the layers. The free end of the beam moves to pressurize the ink at the nozzle causing drop emission. Recently, disclosures of a similar thermo-mechanical DOD ink jet configuration have been made by K. Silverbrook in U.S. Pat. Nos. 6,067,797; 6,087,638; 6,209,989; 6,234,609; 6,239,821; and 6,247,791. Methods of manufacturing thermo-mechanical ink jet devices using microelectronic processes have been disclosed by K. Silverbrook in U.S. Pat. Nos. 6,180,427; 6,254,793; 6,258,284 and 6,274,056. The term xe2x80x9cthermal actuatorxe2x80x9d and thermno-mechanical actuator will be used interchangeably herein.
A useful design for thermo-mechanical actuators is a layered, or laminated, cantilevered beam anchored at one end to the device structure with a free end that deflects perpendicular to the beam. The deflection is caused by setting up thermal expansion gradients in the layered beam, perpendicular to the laminations. Such expansion gradients may be caused by temperature gradients among layers. It is advantageous for pulsed thermal actuators to be able to establish such temperature gradients quickly, and to dissipate them quickly as well, so that the actuator will rapidly restore to an initial position. An optimized cantilevered element may be constructed by using electroresistive materials which are partially patterned into heating resisters for some layers.
A dual actuation thermal actuator configured to generate opposing thermal expansion gradients, hence opposing beam deflections, is useful in a liquid drop emitter to generate pressure impulses at the nozzle which are both positive and negative. Control over the generation and timing of both positive and negative pressure impulses allows fluid and nozzle meniscus effects to be used to favorably alter drop emission characteristics.
The spatial pattern of thermal heating may be altered to result in more deflection for less input of electrical energy. K. Silverbrook has disclosed thermal actuators which have spatially non-uniform thermal patterns in U. S. Pat. Nos. 6,243,113 and 6,364,453. However, the thermo-mechanical bending portions of the disclosed thermal actuators are not configured to be operated in contact with a liquid, rendering them unreliable for use in such devices as liquid drop emitters and microvalves. The disclosed designs are based on coupled arm structures which are inherently difficult to fabricate, may develop post-fabrication twisted shapes, and are subject to easy mechanical damage. The thermal actuator designs disclosed in Silverbrook ""113 have structurally weak base ends which are subjected to peak temperatures, possibly causing early failure.
Further, the thermal actuator designs disclosed in Silverbrook ""453 are directed at solving an anticipated problem of an excessive temperature increase in the center of the thermal actuator, and do not offer increased energy efficiency during actuation. The disclosed actuator designs have heat sink components which increase undesirable liquid backpressure effects when used immersed in a liquid, and, further, add isolated mass which may slow actuator cool down, limiting maximum reliable operating frequencies.
Cantilevered element thermal actuators, which can be operated with reduced energy and at acceptable peak temperatures, and which can be deflected in controlled displacement versus time profiles, are needed in order to build systems that can be fabricated using MEMS fabrication methods and also enable liquid drop emission at high repetition frequency with excellent drop formation characteristics.
It is therefore an object of the present invention to provide a thermo-mechanical actuator which uses reduced input energy and which does not require excessive peak temperatures.
It is also an object of the present invention to provide an energy efficient thermal actuator which comprises dual actuation means that move the thermal actuator in substantially opposite directions allowing rapid restoration of the actuator to a nominal position and more rapid repetitions.
It is further an object of the present invention to provide an energy efficient cantilevered thermal actuator which is actuated by heat pulses having a spatial thermal pattern wherein the base end increases to a higher temperature than the free end of a thermo-mechanical bender portion.
The foregoing and numerous other features, objects and advantages of the present invention will become readily apparent upon a review of the detailed description, claims and drawings set forth herein. These features, objects and advantages are accomplished by constructing a thermal actuator for a micro-electromechanical device comprising a base element and a cantilevered element including a thermo-mechanical bender portion extending from the base element and a free end tip which resides in a first position. The thermo-mechanical bender portion has a base end adjacent the base element and a free end adjacent the free end tip. Apparatus adapted to apply a heat pulse directly to the thermo-mechanical bender portion is provided. The heat pulses have a spatial thermal pattern which results in a greater temperature increase of the base end than the free end of the thermo-mechanical bender portion. The rapid heating of the thermo-mechanical bender portion causes the deflection of the free end tip of the cantilevered element to a second position.
The features, objects and advantages are also accomplished by constructing a thermo-mechanical bender portion which includes a barrier layer constructed of a dielectric material having low thermal conductivity, a first deflector layer constructed of a first electrically resistive material having a large coefficient of thermal expansion, and a second deflector layer constructed of a second electrically resistive material having a large coefficient of thermal expansion wherein the barrier layer is bonded between the first and second deflector layers. A first heater resistor is formed in the first deflector layer and adapted to apply heat energy having a first spatial thermal pattern which results in a first deflector layer base end temperature increase, xcex94T1b, in the first deflector layer at the base end that is greater than a first deflector layer free end temperature increase, xcex94T1f, in the first deflector layer at the free end. A second heater resistor is formed in the second deflector layer and adapted to apply heat energy having a second spatial thermal pattern which results in a second deflector layer base end temperature increase, xcex94T2b, in the second deflector layer at the base end that is greater than a second deflector layer free end temperature increase, xcex94T2f, in the second deflector layer at the free end. A first pair of electrodes is connected to the first heater resistor to apply an electrical pulse to cause resistive heating of the first deflector layer, resulting in a thermal expansion of the first deflector layer relative to the second deflector layer. A second pair of electrodes is connected to the second heater resistor portion to apply an electrical pulse to cause resistive heating of the second deflector layer, resulting in a thermal expansion of the second deflector layer relative to the first deflector layer. Application of an electrical pulse to either the first pair or the second pair of electrodes causes deflection of the cantilevered element away from the first position to a second position, followed by restoration of the cantilevered element to the first position as heat diffuses through the barrier layer and the cantilevered element reaches a uniform temperature.
The present inventions are particularly useful as thermal actuators for liquid drop emitters used as printheads for DOD ink jet printing. In these preferred embodiments the thermal actuator resides in a liquid-filled chamber that includes a nozzle for ejecting liquid. The thermal actuator includes a cantilevered element extending from a wall of the chamber and a free end residing in a first position proximate to the nozzle. Application of an electrical pulse to either the first pair or the second pair of electrodes causes deflection of the cantilevered element away from its first position and, alternately, causes a positive or negative pressure in the liquid at the nozzle. Application of electrical pulses to the first and second pairs of electrodes, and the timing thereof, are used to adjust the characteristics of liquid drop emission.